Aptimil causing constipation, should I change formula? If so, to what??

Hi after breast feeding for 7 weeks I have now switched to formula. I chose Apatamil as my first son had this and we never had a problem plus I believed it to be the best but my Ds is really struggling and in quite a bit of pain until he does his poo. I think I have heard Aparamil can cause constipation so what formula does anyone recommend I switch too?